1. Field of the Invention
The present invention relates generally to a rocket engine, and more specifically to an expander cycle rocket engine with high pressure combustion.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Rocket engines that use cryogenic fuels and oxidizers such as a gas generator cycle and the staged combustion cycle rocket engines use some of the fuel and oxidizer to pre-burn and drive the turbo-pumps that deliver the high pressures to the engine nozzle. In the gas generator cycle, a very small portion of the fuel and the oxidizer is bled off from that delivered to the main combustion chamber (MCC) and diverted into a small pre-burner and combusted to produce a hot gas flow that is then used to drive the turbo-pumps that supply the high pressure fuel and oxidizer to the main combustion chamber. The exhaust gas from the turbo-pumps is then vented overboard. In the staged combustion cycle, a small portion of the propellant (either the oxidizer or the fuel) is diverted and combined with all of the other propellant to partially combust the combination, which is then passed through the turbo-pumps to drive these. This mixture is then sent to the main combustion chamber along with the remainder of the propellant and is combusted in the main combustion chamber. In both the gas generator and staged combustion cycles, some of the fuel and oxidizer is used to produce power to drive the turbo-pumps and therefore not used to produce power in the rocker engine nozzle. Also, because of the high turbo-pump inlet temperature, the turbine driving the turbo-pump is subject to thermal shock and thermal mechanical failure, or TMF.
An expander cycle rocket engine passes a propellant (typically the fuel) through a heat exchanger formed within or around the nozzle to transfer heat from the combustion process in the nozzle to heat up the fuel. The heated fuel (in the case of most cryogenic fuels and oxidizers is hydrogen) is passed through the turbine that drives both of the turbo-pumps to pressurize the fuel and the oxidizer prior to injection into the main combustion chamber for combustion. The expander cycle is more efficient than either of the gas generator and staged combustion cycles described above because all of the fuel and oxidizer is used in combustion and exhausted through the throat and then into the nozzle for expansion. The expander cycle rocket engine has many advantages over the staged combustion and gas generator cycles. Rather than using a pre-burner, the engine routes liquid propellant from the pump discharge to the nozzle. This flow cools the nozzle and heats up the liquid turning it into a gas. The high pressure gas is then routed to the turbine inlet to drive the turbo-pump(s). The turbine is driven by gas expanded from heat transfer in the engine nozzle rather than from products of combustion from a pre-burner as used in the gas generator and staged combustion engine cycles. As a result, the turbine temperature is significantly lower than for the other cycles resulting is longer life due to the elimination of thermal shock and thermal mechanical fatigue (TMF). The expander cycle rocket engine has proven to be the most reliable engine and has demonstrated superior re-start capability. However, in prior art expander cycle rocket engines, the thrust this engine is capable of producing has reached a maximum limit. As the size of the nozzle increases, the engine mass flow increases at a greater rate than the surface area of the nozzle. As a result, a limit is reached when there is insufficient heat transfer in the nozzle to drive the turbo-pump(s) required to provide the mass flow to the engine. Additionally, for a given mass flow, the chamber pressure is also limited based on the turbine power available for driving the turbo-pumps.
High thrust (in excess of 100,000 pounds) expander cycle rocket engines have traditionally been limited to a chamber pressure below 1,500 psia because of a lack of turbine power available to the fuel turbo-pump. In a typical expander cycle rocket engine, fuel from the fuel turbo-pump is pumped through the cooling liner and tubular nozzle of the engine's nozzle assembly where the fuel is heated and then fed to a turbine which drives the turbo-pumps. In order to increase the combustion chamber pressure, flow to the combustion chamber must be increased. However, as fuel flow through the cooling liner and tubular nozzle increases, the temperature of the fuel at the turbo-pump turbine inlet decreases due to the increase in mass flow rate of the fuel or to provide higher discharge pressure. At the same time, the fuel turbo-pump must do more work to provide the increased mass flow rate of the fuel. Although the energy available to the fuel turbo-pump turbine is a function of both the mass flow rate of the fuel and the turbine inlet temperature, the increase in the mass flow rate of fuel cannot offset the resulting decrease in turbine inlet temperature which occurs as a result of the increased fuel flow rate. Consequently, the decrease in turbine inlet temperature and the increase in work required by the turbo-pump at the higher fuel flow rates act to limit the maximum fuel flow rate to the combustion chamber, thereby limiting combustion chamber pressure.
In summary, the expander cycle rocket engine uses heat from the nozzle to heat up the fuel to drive the turbo-pumps that pressurized the fuel and oxidizer for combustion in the nozzle (combustion chamber). To increase the thrust of the rocket engine, a larger propellant flow and/or discharge pressure is required. As the engine/nozzle size increases, the propellant volume increases faster than the surface area of the nozzle. As the nozzle volume increases and more fuel and oxidizer is needed to be pressurized, the amount of heat transferred to the fuel for driving the turbo-pumps becomes less than required to supply the higher pressures. As a result of increasing the nozzle volume, the efficiency of the expander cycle rocket engine becomes less and less. There is a limit in nozzle size using the present technologies because of this effect.
The maximum thrust in a rocket nozzle occurs when the exhaust pressure of the nozzle is equal to the outside pressure of the nozzle. As a result, the rocket thrust rises with increasing altitude.
In the prior art expander cycle engine, the amount of heat generated is limited by the size of the nozzle. The problem lies with the square-cube rule. Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of the bell-shaped nozzle increases with increasing thrust, the nozzle surface area (from which heat can be extracted to expand the fuel) increases as the square of the radius. However, the volume of fuel that must be heated increases as the cube of the radius. Thus, there exists a maximum engine size of approximately 300 kN of thrust beyond which there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps.